The present invention relates to outlet temperature control of an oxidation catalyst and particularly, but not exclusively, to outlet temperature control of an oxidation catalyst for use with a particulate filter regeneration system in diesel vehicle engines.
Regeneration of Diesel Particulate Filters (DPF) requires high exhaust gas temperatures (>550° C.), which are not usual during normal operation of a diesel passenger car. In order to obtain such temperatures, a late injection of fuel in an engine cycle, called a post fuel injection, into the exhaust phase is used to introduce unburned fuel into a Diesel Oxidation Catalyst (DOC) to generate an exothermal reaction. As the DOC is located before the DPF, the increase in temperature of exhaust gas caused by the exothermal reaction in the DOC burns off particulate material in the DPF, enabling additional particulate material to be filtered. In order to reduce thermal stress on the DPF, the outlet temperature of the DOC (inlet of DPF) needs to be controlled to substantially around a target value.
Control of the outlet temperature of the DOC is usually performed by a closed loop controller, such as a standard PID (Proportional-Integral-Derivative) controller, using a temperature sensor at the DOC outlet, the amount of post fuel injection being selected from a table which gives a function of both engine speed and engine load, as well as some compensations for transient conditions.
This type of control has some disadvantages:
it is a non-linear system as the amount of post fuel injection is not proportional to the temperature at the DOC outlet;
variations of exhaust manifold temperatures and/or DOC inlet temperatures are not taken into account, which leads to lack of accuracy of the open-loop term;
variations of ambient conditions may not be taken into account;
delays introduced by the operation of DOC may amplify errors in closed-loop operation;
transient operating conditions, such as variations in speed or load of the engine, usually give different DOC outlet temperatures than corresponding steady state conditions for same speed and load;
any change in the calibration of EGR (Exhaust Gas Recirculation) rate, post injection timing, exhaust flow (throttling), requires a complete recalibration of the post fuel table; and
the only way to change the DOC outlet target temperature is to duplicate the calibration, that is, one complete calibration for 600° C. and a further complete calibration for 550° C.